Synthetic rubber with a very wide variety of different chemical structures is prepared by a very wide variety of polymerization processes and treatment processes. Ullmann's Encyclopaedia of Industrial Chemistry, VCH Weinheim, 1993, Vol. A23, pp. 239 et seq., and references cited therein.
A feature shared by most of these synthetic rubbers is that they have a high molecular weight in conjunction with a broad molecular weight distribution. This broad molecular weight distribution is a consequence of the fact that the great majority of synthetic rubbers is obtained in a free-radical polymerization process which generally gives polydispersity indices D=Mw/Mn, where Mw is the weight-average molecular weight and Mn is the number-average molecular weight, greater than 1.5, and indeed normally in the case of emulsion rubbers greater than 3.0. The polydispersity index D (method of determination: gel permeation chromatography (GPC) against polystyrene equivalents) gives information about the breadth of molecular weight distribution.
Anionic polymerization in solution in principle gives narrower molecular weight distributions, usual polydispersity indices D here being only a little above 1.0, those of commercially obtainable polymers, e.g. solution styrene-butadiene rubber (LSBR) being typically around the value D=2.0. However, this process is applicable only to a small number of the numerous synthetic rubbers with different chemical structures.
The process known as “living radical polymerization”, essentially developed in the last decade, is intended for preparation of narrowly distributed polymers by a free-radical-initiated process. However, apart from a few exceptions there are still no commercial applications of this technique, which is complicated when compared with standard processes. An overview of the current prior art is given by Matyjaszewski (Advances in Polymer Science, Vol. 159, Springer-Verlag Berlin Heidelberg 2002, pp. 2-166).
In the rubber sector, mouldings produced from narrowly distributed polymers have a better property profile when compared with conventional components. This is attributable to a more homogeneous network having a smaller number of what are known as loose ends. This becomes particularly marked on comparison of narrowly distributed solution SBR prepared using an anionic technique with the corresponding product from free-radical polymerization (emulsion SBR).